This invention relates generally to power generation involving the combustion of gas fossil fuels, and more particularly to methods and apparatus for reducing visible emissions in heavy-duty gas turbine power generators.
The condition of a visible yellow plume emanating from the stacks of combined cycle power plants is typically caused by an NO2 concentration exiting the gas turbine engine. As gas turbines have became more sophisticated, exhaust temperatures have increased. The NO2 level exiting the stack comes generally from two sources, NO2 formed in the combustion system and conversion of NO to NO2 in the exhaust path. A higher exhaust temperature increases the amount of NO to NO2 conversion in the exhaust path. Water injection has been shown to facilitate lowering the amount of NOx produced in a combustion system. Typically, water is used in non-Dry Low NOx combustors to reduce NOx over a contractual guarantee range (usually 50%-100% load). In comparison, existing Dry Low NOx combustion systems utilize a diffusion stabilized, partially premixed flame without water injection from 0% to 50% load. Typically, the design requirements for a dry low NOx (DLN) combustion system operating at part load are to maintain a stable flame with robust operability and durability. To mitigate visible exhaust emissions, additional parameters are needed to be added to such list of design requirements. A diffusion stabilized partially premixed DLN combustion flame is not capable of meeting all of these design requirements.
During a cold start of a gas turbine engine, at least some known gas turbine engines are operated for approximately four hours or more at partial loading (0 to 50%) prior to reaching full pre-mixed operation (50 to 100% load). During this partial loading operation, the level of NO2 exiting the exhaust stack can be sufficient to cause a visible plume. It has been found that the concentration of NO2 exiting the stack can be up to three times the concentration of NO2 exiting the gas turbine, while the overall NOx level remained substantially constant. The relationship, NOx=NO+NO2 suggests that conversion of NO to NO2 occurs between the gas turbine exit and the top of the exhaust stack. The amount of NO2 exiting the exhaust stack, which can appear as a yellow plume, does not violate any environmental regulations, however, it is perceived as unsightly and can lead to a poor public perception of the power plant.